In the manufacture of semiconductor devices, photolithography is often used. Projection optics are used to image a mask or reticle onto a wafer. Optical systems having a refractive group have achieved satisfactory resolutions operating with illumination sources having wavelengths of 248 or 193 nanometers. As the element or feature size of semiconductor devices becomes smaller, the need for optical projection systems capable of providing enhanced resolution are needed. In order to decrease the feature size which the optical projection systems used in photolithography can resolve, shorter wavelengths of electromagnetic radiation must be used to project the image of a reticle or mask onto a photosensitive substrate, such as a semiconductor wafer.
Because very few refractive optical materials are able to transmit significant electromagnetic radiation below a wavelength of 193 nanometers, it is necessary to reduce to a minimum or eliminate refractive elements in optical projection systems operating at wavelengths below 193 nanometers. However, the desire to resolve ever smaller features makes necessary optical projection systems that operate at the extreme ultraviolet wavelengths, below 200 nm; and therefore, as optical lithography extends into shorter wavelengths (e.g., vacuum ultraviolet (VUV)), the requirements of the projection system become more difficult to satisfy.
It has long been realized that catadioptric or catoptric optical systems have several advantages, especially in a step and scan configuration, and that catadioptric or catoptric systems are particularly well-suited to satisfy the aforementioned objectives. A number of parties have developed or proposed development of systems for wavelengths below 365 nm.
In a typical arrangement, a projection optics box (POB) contains the optical components that are used to reduce the image and form it on the photosensitive substrate (wafer). In most projection optical systems, mirrors that are carefully crafted to perform the intended functions are used in combination with a number of lenses arranged relative thereto. The mirrors serve to redirect the light in the projection optic box as it passes therethrough from the mask to the photosensitive substrate. Typically, the POB includes an arrangement of mirrors and/or lenses that are constructed and positioned to accomplish the intended result. U.S. patent application publication No. 2003/0058422 discloses a lithographic projection apparatus includes a projection system having a plurality of optical elements or sensors mounted on a frame and U.S. patent application publication No. 2003/0010902 discloses an optical system, in particular an exposure lens for semiconductor lithography, with a plurality of optical elements has at least one load-dissipating structure. The load-dissipating structure diverts the forces originating from the optical elements. The optical system also has a measuring structure constructed independently of the at least one load-dissipating structure. Exemplary components and interfaces for positioning of mirrors in catadioptric systems have been the subject of a number of patent applications filed by the present assignee and include U.S. patent application Ser. No. 10/704,534, which is directed to hermetically sealed elements of an actuator.
Conventional mirror mounts that have been used as interface members between the mirror(s) and actuators, which serve to move and position the mirrors, suffer from a number of deficiencies. For example, the mirror mounts do not strike the proper balance between offering a stiff connection while providing a decoupling of forces (e.g., radial forces) and moments. In other words, the conventional mirror mounts fail to minimize the forces that are transferred to the mirror. This leads to the mirror mounts deforming of the mirror during normal operations as well as other performance difficulties.
What has heretofore not been available is a mirror mount that is robust and reliable, while also at the same time minimizes the forces that are transferred to the mirror.